1. Technical Field
The present disclosure relates to a control integrated circuit for a power transistor of a switching current regulator.
The proposed control integrated circuit is more directly employed in offline converters, and in particular in those employing the flyback topology.
2. Description of the Related Art
Offline converters typically have a so-called isolation barrier, i.e., including two galvanically separate parts. One so-called primary side, configured to be connected to an electricity power line through a rectifier bridge, usually includes a switch (typically a MOSFET), the opening and closure of which is suitably driven so as to regulate the power flow, and a controller for controlling the switch. A so-called secondary side is isolated from the primary side and connected to a load to be supplied by an output terminal of the secondary side.
The galvanic isolation, specified by safety standards, is ensured by the presence of a transformer. The transformer, configured so as to provide a suitable isolation, established by the legal regulations, allows the energy to pass from one side to the other by magnetic coupling, without metal contact therebetween.
In all converters either the output voltage or the output current is to be regulated, i.e., maintained at constant value as the operating conditions change (input voltage, output load, temperature). In this context the focus is on switching current regulators, then the output current is the quantity to be regulated. This objective is typically achieved by using a feedback control: the output current, or a portion thereof, is compared with a reference value; their difference is suitably amplified (error signal) and processed by a control circuit (controller) in order to determine the turn-on and turn-off time of the switch so as to zero or minimize the aforesaid error signal.
At this point, a problem arises in offline converters: the output current is on the secondary side while the control and the MOSFET are on the primary side. Therefore, the error signal should be transferred from one side to the other, i.e., the isolation barrier should be crossed in the opposite direction and, according to the safety regulations, the same isolation as the transformer should be at least ensured. A solution to this problem consists in using another small transformer or an optocoupler.
However, due to cost problems, it is desirable to regulate the output current without using a feedback loop; in such a case the optocoupler is no longer used.
A flyback converter which is provided with a regulation of the output current operated on the primary side of the transformer is described in U.S. Pat. No. 5,729,443. The flyback converter, shown in FIG. 1, comprises a sensor 20 which detects the current flowing in the power transistor S (indicated by a switch) connected to the primary winding 11, in turn connected to the input voltage V1. The flyback converter also includes a set-reset flip-flop 22 the output Q of which controls the power transistor S, the reset input R of which receives the output of a comparator 24 and the set input S of which receives the output of a demagnetization detection device 26 connected to a terminal of the primary winding. The comparator 24 is adapted to compare the detected voltage K*I1 with a reference voltage Vr. When the power transistor S is on, the current I2 on the secondary 12 of the transformer is null, as seen in FIG. 2. When the detected voltage K*I1 reaches the reference voltage Vr, the comparator 24 resets the flip-flop 22 which turns the transistor S off and the current I1 becomes null. The current I2 reaches its peak and then regularly decreases until reaching zero, which indicates the demagnetization of the magnetic core. The demagnetization is detected by the circuit 26 which sends the set signal to the flip-flop 22 to turn the transistor S on.
The flyback converter comprises circuitry coupled to the power supply voltage Vcc and adapted to vary the reference voltage Vr with the variations of the switching duty cycle. The circuitry includes a capacitor Cr having a first terminal connected to ground GND and a second terminal at which the reference voltage Vr is produced and provided to the inverting input of the comparator 24. The circuitry also includes a reference current generator, which produces a reference current Ir, connected between the power supply voltage Vcc and the second terminal of the capacitor Cr. The capacitor Cr is arranged in parallel to a series of a resistor Rr and a switch S1 connected to ground GND and controlled by the output Q*, i.e., the negated output Q of the flip-flop 22. FIG. 2 shows the time diagrams of the signals S, Vr, KI1 and I2.
Therefore, said control device operates with a continuous input voltage and does not work properly in the case of a rectified input voltage, such as in the case of a flyback converter with high power factor, i.e., higher than 0.9.